It is often necessary or desirable to nondestructively determine whether a painted metallic surface has been painted more than once or whether there exits between the metal surface and the paint layer another material, such as a body filler. This information is useful, for example, in determining the condition and value of used motor vehicles. Revelation of the presence of several paint layers or body fillers on some portion of a used car body would undoubtedly indicate previous damage and subsequent repair to that portion of the vehicle, although no such history would be revealed by mere visual observation of the outside of the vehicle.
One of the prior methods of determining the thickness of layered targets consisting of a number of dielectric layers involves the use of electromagnetic waves having related frequencies such as a fundamental and its harmonics to establish a multi-harmonic coherence relationship whereby a homodyne phase reference between harmonics can be conserved and information can be extracted from just the received and not the transmitted signals. Such a device is disclosed in U.S. Pat. No. 4,075,555 to James S. Wight, et al. In this arrangement, the necessary hardware to extract the phase is relatively complicated, expensive and does not always result in apparatus which may be handheld or which is portable. Furthermore, the resolution of the measurement is rated in fractions of a centimeter or several millimeters, which is far too low and completely inadequate for the purposes of the present invention. Frequently, devices of this general type, as disclosed in U.S. Pat. No. 5,216,372 to Zoughi et al. use multiple diode detectors in a waveguide separated from each other by very specific distances. Such multiple spaced diodes must be calibrated for the differences in the power-in, voltage-out characteristics which may change during the operation, making the approach limited to stationary devices, as opposed to easily portable ones.
Another device directed to apparatus for measuring properties of materials by sensing signals responsive to both amplitude and phase changes in transmitted or reflected microwave energy is disclosed in U.S. Pat. No. 3,562,642 issued to Richard Hochschild. The teachings of this disclosure are not pertinent to the invention herein described for many reasons, but Hochschild's use of multiple detectors is one distinction.
U.S. Pat. No. 4,818,930 to Flemming, et al. disclosed apparatus for measuring the thickness of a thin layer of a non-metallic material on the surface of another medium, however this device requires the transmission of an incident beam at an angle of incidence greater than 30 degrees and the detection of polarized reflections, neither of which is a part of the invention to be described.
To the knowledge of the inventors there are no other prior art disclosures which would be pertinent to the invention which is the subject of the disclosure made herein.
It is therefore the primary object of the present invention to provide a coatings layer counter for layers of paint (or other dielectric layered materials) applied to metallic or other electrically conductive surfaces which requires only one detector diode and is otherwise simple enough in its design and construction to be hand held and modest enough in its theory of operation that it can successfully determine the layer count with required resolution by measuring the magnitude of only one voltage component of a standing wave of microwave energy.
Another object of the invention is to provide a device of the character described which will be relatively inexpensive and yet reliable and will have good repeatability and sufficient resolution to detect at least one half of a normal paint layer, or better.
A further object of the invention is to provide a microwave transmitting and receiving apparatus which requires no antenna and one which will operate as a near field measuring device, as opposed to a far field device.
Yet another object of the invention is to provide a microwave instrument for measurement using only a fixed diode location which depends for its success only on the magnitude of the standing wave at a specific location in the waveguide to obtain the measurement data instead of detecting phase relationships of received microwave signals.
Other and still further object, features and advantages of the present invention will become apparent upon a reading of the following detailed description of a preferred form of the invention.